Anatomical and functional heterogeneity of the medullary respiratory neurons

In a systematic study we attempted to activate the respiratory neurons antidromically by stimulating the spinal cord and the vagus. This has led to an anatomical and functional classification of the neurons: bulb-spinal neurons whose axons enter the spinal cord, vagal motoneurons, propriobulbar neurons which cannot be activated antidromically. We used simultaneously other tests (localization, response to Hering-Breuer reflexes or to cortical stimulation) which gave positive results. Two groups of respiratory neurons were apparent: a dorsal respiratory nucleus with only inspiratory bulbo-spinal neurons and a ventral respiratory nucleus with inspiratory and expiratory bulbo-spinal neurons, vagal motoneurons and propriobulbar neurons. The bulbo-spinal neurons and some propriobulbar neurons are normally related to Hering-Breuer reflexes evoked by lung inflation or deflation, but the recurrent laryngeal motoneurons or fibres and some propriobulbar neurons can give a paradoxical response to lung inflation. By cortical stimulation the propriobulbar neurons show only an arrest of their discharge, whereas the bulbo-spinal neurons give a burst of spikes followed by an inhibition. These results led to the following conclusions: (i) the propriobulbar neurons do exist; (ii) some neurons classified as medullary respiratory neurons have not really a ventilatory function: they belong to the laryngeal system; (iii) it is fundamental to use systematically the antidromic test and some other ones to know exactly the hodological situation of the neurons in the respiratory complex.